Bounceless high pressure drop cascade impactor and a method for determining particle size distribution of an aerosol

ABSTRACT

This invention is directed to a high pressure drop cascade impactor wherein the particles do not bounce from the collection plate; to a method for designing said impactor; a method for calculating and determining the particle size distribution of an aerosol wherein the particle size may have a Stokes (aerodynamic) diameter as small as 10 nanometers; and to a method of manufacturing such an impactor. 
     The impactor that is the subject of this invention is different from and superior to previously described impactors in the following principal respects: 
     1. The particles exhibit substantially no reentrainment, or bounce, from the collection surfaces. This property is achieved by (a) limiting the exit jet stream velocities such that the herein defined &#34;bounce parameter&#34;, β, is never exceeded, and (b) increasing the thickness of the jet plates to elongate the jet passageways in order to reduce the jet velocities so that bounce will not occur. 
     2. The impactor described herein eliminates turbulence near the entrances to the jet holes by rounding the edges of the jet holes. This is accomplished on the small holes by means of electropolishing. 
     3. The impactor described herein is capable of capturing particles as small as 40 nanometers Stokes (aerodynamic) diameter. By contract, previously described impactors could capture particles only as small as 300 nanometers Stokes diameter.

This application is a continuation-in-part of my co-pending patentapplication, Ser. No. 464,158, and filing date of Apr. 25, 1974, nowabandoned.

THE GENERAL BACKGROUND OF THE INVENTION

In the atmosphere around cities and industrial plants, there are manysmall particles of matter.

An aerosol may be defined as a group of solid particles or liquidparticles suspended in a gaseous medium. The size range of theseparticles is generally between 10 nanometers and 100,000 nanometers indiameter. In an aerosol, the large particles account for most of themass or weight of the aerosol. From observation, it appears that theparticle sizes between 100 nanometers and 1000 nanometers cause thegreatest health impairment and also cause the greatest decrease ofvisibility in the atmosphere.

Prior to this invention, there was no convenient means or method tomeasure the size distribution for particles in the size range of 10nanometers to 300 nanometers in the atmosphere. If the size of theparticles in an atmosphere could be measured to 10 nanometers, it isreasonable to conclude that means and methods could be found to controlthese fine particles in the atmosphere and to remove these fineparticles so as to lessen the danger to the health.

Examples of pollutants and particulate matter in the atmosphere are theeffluent from a plant burning coal, effluent from an aluminum reductionplant, and the general particulate matter in the atmosphere.

The effluent from a plant for burning coal comprises particulate matterranging in size, as determined by a cascade impactor prior to thisinvention, of particles having a diameter in the range of 300 nanometersto 10,000 nanometers. The cascade impactors prior to this invention werenot capable of measuring the particle size to a diameter less than 300nanometers. The effluent from a coal burning plant comprises a,relatively, wide reange of particle sizes. The larger particles of theatmosphere, relatively, close to the coal burning plant while thesmaller particles will settle out of the atmosphere at a greaterdistance from a coal burning plant. And, the smallest particles in theeffluent will not settle out from the atmosphere but will be washed outof the atmosphere by rain and snow and the like. It is my understandingthat, the particle sizes between 50 nanometers and 1000 nanometers posethe greatest problems to health. For example, at the present time, it isbelieved that the particles having a diameter in the range of 50nanometers to 1000 nanometers pose the greatest health hazard and theparticles having a diameter in the range of 100 nanometers pose thegreatest bisibility problems. The particulate matter in the effluentfrom a coal burning plant poses problems with respect to determining thesize of the particulate matter and also in removing the particulatematter from the effluent. At the present time, one of the biggestproblems is the determination of the size of the particulate matter inthe effluent.

Another example is the size of the particulate matter in the effluentfrom an aluminum reduction plant. As is well known, in an aluminumreduction plant there are used electrodes. The electrodes are made froma paste of carbon particles in a hydrocarbon matrix. For example, theanode may be formed in the Soderberg process by continually adding pasteand letting the hydrocarbon bake or heat and cook to form an anode. Inthe formation of the anode, there is given off a large amount ofhydrocarbons. Or, the anode may be formed in a separate facility so asto be a prebaked anode and then inserted into the potline for making themolten aluminum. In the facility for prebaking the anode, there is alsogiven off a large amount of hydrocarbon. The hydrocarbons are given offinto the atmosphere and, because of a nucleation process taking place inthe atmosphere, are condensed to form a haze, such as a typical bluehaze. At the aluminum reduction plant at Tacoma, Washington, theeffluent from the plant was measured by a cascade impactor, prior to thecascade impactor of this invention, and the particle size ranged from adiameter of 300 nanometers to 10,000 nanometers. There is scientificreason to believe that in the effluent from the aluminum reductionplant, there were many particles of a diameter of less than 300nanometers, but the capacity of the prior cascade impactor was notsufficient to capture and weigh a particle size less than 300nanometers. The comments with respect to the particle size distributionin the effluent from the coal burning plant are applicable to theparticle size distribution in the effluent from the aluminum reductionplant with respect to posing a health hazard and to posing a visibilityhazard. Further, it is known that in the effluent from a Soderbergaluminum reduction plant that the effluent contains 3-, 4-benzopyrenewhich is a carcinogen and hazardous to the health of individuals.

In Seattle, Washington area, the particle size distribution in theatmosphere for Mar. 17, 18 and 19, 1966, was determined by capturing theparticles by means of a thermal precipitator on a glass plate and, thenby means of an electron microscope, determining the size of theparticles captured. The size distribution ranged from 10 nanometers to1000 nanometers. This is a typical particle size range for aerosolsgenerated in the atmosphere.

The small particles in an aerosol may be the result of a comminutionprocess whereby erosion reduces the size of a particle to form thesmaller particle. An example is the grinding of metal, the rubbingtogether of solid material, the blowing of wind on rock, and manycrushing and grinding operation that are common in industry. Another wayof forming the small particles is the atmosphere is by a nucleationprocess whereby gases can condense or react to form tiny liquid or solidparticles. After these particles have nucleated, they grow by coalescingwith one another and/or by gas condensing on the particles to formlarger particles. As a generalization, particles formed by thenucleation process are less than about 300 nanometers in diameter andparticles formed by the comminution process are greater than about 300nanometers in diameter.

There are means and methods for measuring particles having a diameterless than 1000 nanometers. One of these is the Aiken Counter which iscapable of measuring the number of particles having a size less than 100nanometers. The size of the particle itself is not measured by the AikenCounter but the number of particles below 100 nanometers in diameter aremeasured. Also, there is a question in regard to the interpretation ofthe results of the Aiken Counter. This introduces a question of thereliability of the results of the Aiken Counter.

Another means is the diffusion cell for measuring the particle size. Inthe use of the diffusion cell, it is necessary to have an individualcell for each range of particle size. This means that the diffusion cellprocess is an expensive process and also a tedious process to use. Withthe diffusion cell, it is possible to measure a particle size in therange of about 10 nanometers in diameter.

Another means is the combination of a thermal precipitator and anelectron microscope wherein the thermal precipitator captures all theparticles and with the aid of the electron microscope the size of thecaptured particles can be determined. The means for capturing anddetermining the size of the particles is expensive and the process is aslow process.

Another means is a cascade impactor. In a cascade impactor, there are anumber of stages, each stage comprised of a jet plate and a collectionplate. As the aerosol-laden gas passes through the impactor, the gas iscaused to pass through each jet plate and impinge on the correspondingcollection plate. The gas velocity in each jet stage is higher than thevelocity in the preceding stage. As the gas passes from stage to stage,each collection plate collects a smaller size range of particles thanwas collected by the preceding stage. The collection plates are weighedbefore and after the sampling period to determined the weight collectedby each stage. In using a cascade impactor, the impactor is calibratedso that the particle size range captured on each plate is known. It isthen possible, by means of the weight of the particles captured on theplate and the particle size range, to state the percent of particles byweight in a given stage or on a given plate. One of the disadvantages ofthe prior cascade impactors has been that a hard particle in an aerosol,such as fly ash from a coal burning plant, will bounce on the plate. Afurther disadvantage of a cascade impactor, prior to this invention, hasbeen that a particle size less than 200 nm in diameter has not beencaptured except for research models operating at very low inletpressure. A particle having a diameter less than 200 nm flows throughthe cascade impactor and is not captured for measurement anddetermination. In certain instances, particles having a size less than200 nm have flowed through the cascade impactor and have been capturedon a filter. The filter has been weighed so it is possible to know theaggregate weight of the particles having a diameter less than 200 nm butit has not been possible to determine the size of these particles.

THE GENERAL DESCRIPTION OF THE INVENTION

The invention is a cascade impactor comprising a plurality of collectorplates positioned between jet plates. A gas containing solids and/orliquids, an aerosol, is drawn into the impactor and certain of thesesolids and/or liquids impact upon a collector plate and are entrapped onthe collector plate.

The velocity of the gas containing the liquid and/or solid increases inflowing through the impactor. Initially, the velocity of gas is,relatively, slow so that the larger particles contact the collectorplate and stay on the collector plate. Then, the gas flows through a jetplate and the velocity increases slightly, and the particles in the gascontact the next collector plate and some of these particles remain onthe next collector plate. Now, this process is repeated many times ingoing from a jet plate to the next succeeding collector plate. In goingfrom the preceding jet plate to the succeeding jet plate, the velocityincreases and the particles collected on the collector plate decrease insize. As is readily appreciated, the series of successive collectorplates function to collect smaller size particles.

The velocity of the gas and particles depend upon the hole size and thenumber of holes in the jet plate. The succeeding jet plates have smallerhole sizes than the preceding jet plates until the minimum practicalhole size is reached. Then, the hole size remains the same for a numberof jet plates. For the last one or two or three jet plates, the holesize may increase to accomodate the larger volume of gas of the latterstages, which results from gas expansion through the impactor.

The collector plates comprise a substrate or a substrate liner. Thesubstrate or substrate line may be foil such as aluminum foil or may bea plastic. The substrate liner is chosen so that the tare weight of theliner is as small as, reasonably, possible. It is called to theattention of the reader that the weight of particles collected on thesubstrate liner is, relatively, small and the weight of the substrateliner should be small. Also, the substrate liner is weighed before thesample is taken and weighed after the sample is taken so as to determinethe weight of the sample. For example, the substrate liner is weighed ona micro-balance capable of measuring to as small as 1 microgram. Thesubstrate liner may weigh in the range of 100 milligrams. From this, itis seen that it is desirable to have the substrate liner be of a lowweight.

There is terminology used with respect to the cascade impactor known asd₅₀ which symbolizes the diameter at which 50 percent of the particlesare captured on a given stage or on a given collector plate. Theanalyzer, by knowing the weight of the particles captured and the d₅₀value for that particular collector plate, and then the d₅₀ values forall the collector plates, can make a particle size distributioncalculation.

Cascade impactors prior to this invention have had a lower size limit ofabout 200 nanometers. Also, in previous impactors, the hard particlesand high-density particles have bounced off the collector plates. Withthis impactor, the d₅₀ value on the last stage indicates a diameter of40 nanometers. The smallest particle capable of being collected on thisimpactor is in the range of a Stokes diameter of 10 nanometers, whereina Stokes diameter represents the real diameter times the square root ofthe density of the particle being collected on the collector plate. Fromthis, it is to be realized that the Stokes diameter of a high-densityparticle is greater than the real diameter of the high-density particleand thus the Stokes diameter of a particle having a density less thanthe density of water is greater than the real diameter. For example, flyash from a coal burning plant has a relatively high density of about2.5. The real diameter of the fly ash may be in the range of 15nanometers, but the Stokes diameter may be in the range of about 24nanometers.

Further, with this invention, I have tried to eliminate the bounce ofhard and high-density particles from the collector plate. This, Ibelieve, is an accomplishment which has not been realized with othercascade impactors. To overcome the bounce off of the collector plates,other researches have coated the collector plates with an absorptionmaterial such as a grease or an oil or a mat such as a fiberglass mat.The use of such materials is messy and time consuming and with a greaseor an oil there results a weight inaccuracy due to some vaporization ofthe grease and oil from the collector plate. With fiberglass, there isan even more serious problem as some of the fiberglass will flake offwhen being handled, after the particles have been collected on thecollector plate and cause weight inaccuracies or sill result in aninaccurate weight.

THE OBJECTS AND ADVANTAGES OF THE INVENTION

An object and advantage of this invention is to separate an aerosol intoparticle size distribution fractions by inertial means and not byelectrical means such as condensation nuclei; a further object is toprovide the teaching for making an impactor and which impactor needs tobe calibrated only once and does not have to be calibrated each time itis employed or used; another object is to disclose an impactor which isa simple instrument to make; a further important object is to disclosean impactor which is a rugged instrument; another important object is todisclose an impactor which is a reliable instrument for determiningparticle size distribution in an aerosol and which particle sizedistribution can be repeated; another practical aspect of this inventionis to provide an impactor which is, relatively, inexpensive tomanufacture; a further important object is to provide an impactorwhereby it is easy to train someone to use the impactor and it is notnecessary to have a, highly, formally, educated person to use theimpactor; an additional object is to provide an impactor which makes itpossible to get a particle size distribution of an aerosol wherein thesmallest size particle may have a Stokes diameter in the range of about10 nm; an additional object is to provide an impactor whereby it ispossible to get an accurate particle size distribution of the particlesin an aerosol; a further important object is to provide an impactorwhereby the impactor can be used in on-site testing of an aerosol todetermine the particle size distribution and the impactor is portableand not limited to use as a research tool but can be used in the fieldas a practical operating tool; and, to provide an impactor whereby it isnot necessary to use a medium such as oil or grease or a fiberglass maton the collector plates for capturing and retaining the particles.

A further advantage of the impactor is the self-regulating feature whichwill be described later.

An example of the work that can be done by the impactor of thisinvention is illustrated in FIG. 27 which shows two curves of actualtest data. One curve shows a size distribution for stack gas from a hogfuel boiler. The other curve shows a size distribution for the roof ventmonitor gases from a Soderberg aluminum reduction plant. In both casesthe analyses show considerable detail about the size distribution atparticle diameters less than 1000 nanometers. By comparison, the datawas recalculated to show the amount of information that would have beenobtained from one of the cascade impactors previous to the one that isthe subject of this invention. The Solid black circles on FIG. 27 arethe points that would have been obtained by a previous impactor at sizesless than 1000 nanometers.

With the use of this impactor it has been possible to discover that thepeak concentration of fine particulate (smaller than 2000 nanometersdiameter) is invariably at 300 nanometers. So far as is known, thisphenomena has not been observed before in industrial effluent stacks.Thus a new tool has been made available to scientists and others in thefield of air pollution control. This tool will measure fine particulatesizes at the high concentrations that are encountered in industrialstacks, something that could not be done before.

These and other important objects and advantages of the invention willbe more, particularly, brought forth upon reference to the accompanyingdrawings, the specific description of the invention, and the appendedclaims.

THE DRAWINGS

In the drawings:

FIG. 1 is a fragmentary side elevational view of a specific embodimentof this invention constructed in accordance with the teachings thereof;

FIG. 2 is an end view looking into the entrance of the impactor;

FIG. 3 is an end view looking into the exit of the impactor;

FIG. 4, taken on line 4--4 of FIG. 1, is a longitudinal cross-sectionalview illustrating the details of construction of the impactor and bymeans of lines and arrows the flow pattern of the aerosol-laden gasthrough the impactor;

FIG. 5 is an inside view of the outlet head of the impactor and lookingoutwardly through said outlet head;

FIG. 6 is an inside view of the inlet head of the impactor and lookingoutwardly through said inlet head;

FIG. 7 is a top view of the impactor plate of Stage 1 of the impactorand looking into the inlet side of said impactor plate;

FIG. 8, taken on line 8--8 of FIG. 7, is a longitudinal cross-sectionalview of said impactor plate;

FIG. 9 is a bottom view of the impactor plate of Stage 1 and is lookingat the outlet side or the downstream side of said impactor plate;

FIG. 10 is an inside view looking at a typical jet plate such as used onStage 2 and the like of the impactor and looking into the jet plate orthe upstream side of the jet plate;

FIG. 11, taken on line 11--11 of FIG. 10, is a longitudinalcross-sectional view of said jet plate as used, for example, on Stage 2of the impactor;

FIG. 12 is a plan view looking at the outlet side or the downstream sideof a typical jet plate as used, for example, on Stage 2 of the impactor;

FIG. 13 is a view looking into a collector plate such as used on Stage 2and the like of the impactor;

FIG. 14, taken on line 14--14 of FIG. 13, is a longitudinalcross-sectional view illustrating the collector plate and also thesubstrate placed on the collector plate for collecting the liquid andthe colid in the aerosol-laden gas;

FIG. 15 is a view looking at the outlet or downstream side of thecollector plate such as used on Stage 2 and the like of the impactor;

FIG. 16 is a view looking at the substrate placed on a collector plateand which substrate collects the liquid and the solid particles from theaerosol laden gas;

FIG. 17, taken on line 17--17 of FIG. 16, is a cross-sectional view ofthe substrate;

FIG. 18 is a view looking into a jet plate or looking into the upstreamside of a jet plate, and which jet plate is positioned near the outletend of the impactor, and illustrates the plurality of small holes in thejet plate;

FIG. 19 is a longitudinal cross-sectional view of a jet plate of FIG.18, for a jet stage which has the minimum jet plate thickness (dimensiont);

FIG. 20 is a view of the outlet side or the downstream side of the jetplate of FIG. 18;

FIG. 21 is a longitudinal cross-sectional view of a jet plate of FIG.18, for a jet stage which has a very thick jet plate, for reasons whichwill be explained later;

FIG. 22 is a magnified cut-away portion of FIG. 18 showing accumulationof particles on the jet plate when the jet plate has sharp-edged holes;

FIG. 23 is a cross-sectional view of FIG. 22 on line 23--23, showing theturbulent flow patterns of aerosol laden gas when the jet plate holes inthe jet plate have square edges, and also shown are the accumulations ofparticles on the jet plate caused by the turbulence;

FIG. 24 is a cross-sectional view, similar to FIG. 23, except that thejet plate holes have rounded edges on the upstream side and with gasflow patterns illustrated with the reader's attention called to the lackof a collection of particles around the entrance to the holes and thedifference in flow patterns between FIG. 23 and FIG. 24.

FIG. 25 is a cross-sectional elevational view of a typical jet platewhen it is being polished in an electrolyte to round and upstream edgesof the jet holes;

FIG. 26 is an exploded view illustrating the components of the impactorsuch as the inlet end, the impactor plate, the jet plate, the substrate,the collector plate, and the outlet head;

FIG. 27 is a graphical representation of data illustrated by opentriangles and open circles and solid lines from two sized distributiondeterminations made with an impactor according to the teachings of thisinvention and with the solid black circles and the dashed lines torepresent a calculation of the amount of data what would have beenobtained in these tests if an impactor, other than the impactoraccording to this invention, and been used; and,

FIG. 28 is a graphical representation of the pressure drop curves forthe impactor and for the vacuum pumping system.

FIG. 29 is a drawing of the upstream face of a jet plate which employsthe alternate configuration of slit jets.

FIG. 30 is a cross-sectional view of the slit jet plate along section30--30.

THE DETAILED DESCRIPTION OF THE IMPACTOR

This cascade impactor is comprised of a series of stages. Each stage iscomprised of a jet plate and a collector plate substrate. Each jet platehas one or more holes which direct the gas toward the correspondingcollector plate. The collecting surface of each collector platesubstrate is so positioned so as to be directly in the path of thestream or streams from the corresponding jet plate. When anaerosol-laden gas passes through the cascade impactor it goes throughthe first jet stage, thence is deflected off the first collector platesubstrate, and passes through the second jet stage, and is deflected offthe second collector plate substrate, and so on until the stream isdeflected off the last collector plate and passes to the vacuum pump.

When a gas stream containing an aerosol particle is directed toward aflat plate, the particle will impinge on the plate and remain on theplate, or the particle will be swept aside by the gas stream, dependingupon the size (and hence inertia) of the particle and the velocity ofthe gas stream. In the first jet stage, the velocity of the gas is low,and only the largest particles are captured on the first collectorsubstrate.

The size of the jet holes and the number of jet holes are selected sothat, in general, the velocity of the gas in each jet stage is greaterthan the velocity of the gas in the preceding stage. Each collectingsubstrate, downstream from its respective jet stage, collects a fractionof the aerosol particles that is smaller in size (diameter) than thefraction captured by the preceding collecting substrate. Each collectingsubstrate is weighed before and after the sampling period, so that theweight of particles in each size fraction is determined. From thesedata, a particle size distribution is calculated. Note:

(In certain cases, the velocity of the gas in a jet plate will be lessthan the velocity of the gas in the preceding jet plate. However, theproduct of the gas velocity and the Cunningham slip correction factor(that is, V×C) for any stage will always be greater than said productfor the preceding stage.).

It is to be realized that those particles having a particle size lessthat about 10 nm (or 5 nm in the case of dense particles), continue toflow through the impactor in the aerosol-laden gas. The particles havinga particle size greater than about 10 nm will separate from the gasstream and be impacted on one of the collector plate substrates and beweighed for the size distribution analysis. More particularly, the firststage of the impactor will capture those particles in the gas streamhaving a particle size of about 10,000 nm or greater. Then, thoseparticles in the range of about 10 nm particle size to about 10,000 nmparticle size will be captured on the other collector plates so as toseparate the particles according to their size range on the othercollector plates or substrates so as to realize said particle sizedistribution analysis by weight.

In FIG. 1, there is illustrated in a side elevational view, the impactor30 having an entrance nozzle 32 which is also the first jet stage and anexit fitting 34.

In FIG. 4, it is seen that the entrance nozzle has longitudinalpassageway 36 which is tapped at 38 at its outer end.

In FIG. 4, it is seen that the exit fitting 34 has longitudinalpassageway 40 which is tapped at 42 on its outer end.

The entrance nozzle 32 has a back side 44. In the back side 44 there isa circular groove 46. Further, in the back side 44 and connecting withthe circular groove 46 is a circular groove 48.

The impactor 30 comprises a first collector unit 50 having cylindricalside walls 52 and a first collector plate 54. In FIG. 7 there isillustrated a view looking into the first collector unit 50 and thefirst collector plate 54. In FIG. 9, there is a view looking at the exitside of the first collector unit 50 and the first collector plate 54. InFIGS. 7, 8 and 9, it is seen that around the periphery of the plate 54,there are four passageways 56.

In FIGS. 4, 7 and 8, there is shown a thin substrate 58 overlying theupstream face of the plate for collecting the particulate matter carriedby the aerosol. The substrate 58 may be aluminum or a plastic or othersuitable material.

In FIG. 4, there is illustrated a second jet stage 60 having cylindricalside wall 62 and a second plate 64. In the second plate 64 are a numberof passageways 66 to accelerate the flow of gas through the second plate64 for causing a certain fraction of the particulate matter in the gasto deposit on the next succeeding collector plate. In side wall 62,there is a tapped opening 68. The tapped opening 68 is for receiving theinlet to a pressure sensitive device. When the pressure sensitive deviceis not connected to the impactor 30, there is a bolt 70 in the tappedopening 68. The second jet plate 60 has on its inlet side of the sidewall 62 a surface 72 and in the surface 72 is a circular groove 74.Also, on the outlet side, there is a surface 76 and in the surface 76 isa circular groove 78.

In FIG. 4, it is seen that in the circular groove 48 in the entrancenozzle 32 there is an O-ring 80. In the groove 74, there is an O-ring 82and in the groove 78, there is an O-ring 84.

In FIG. 4, it is seen that there is a second collector unit 86 having acircular side wall 88 and a collector plate 90. In the collector plate90, there is a central passageway 92. It is to be realized that in thesecond collector plate 86 and on the upstream side of the collectorplate 90, there is a thin substrate 96 for collecting the particulatematter in the gas stream and which particulate matter will separate as afraction onto the thin substrate 96. More, particularly, in FIGS. 16 and17, there is illustrated the thin substrate 96 of a generallycylindrical configuration and having a central passageway 98 to allowthe gas to flow through the substrate. The central passageway 98 in thethin substrate 96 will be aligned with the central passageway 92 in thecollector plate 90. Again, the thin substrate 96 may be aluminum orplastic or other suitable material. It is to be recalled that one of thereasons for using the thin substrate 96 is the weighing of the thinsubstrate before it is used to collect particulate matter so as toestablish a tare weight and then to weigh the substrate with theparticulate matter to determine the weight of the particulate matter.With the analytical equipment used, it is desirable to have the thinsubstrate weigh as little as possible so as to have as small a tareweight as possible. In other words, a light weight substrate increasesthe sensitivity of the balance so as to be able to secure a moreaccurate weight of the particulate matter.

When the impactor is used in any position other than the entrance nozzlepointing up, there is a problem of keeping the substrate foil or plasticfilm 96 in position on the upstream face of the collector plate 90. Ifthe foil contacts the downstream face of the jet plate 100 or 170,proper impaction and separation of particles cannot take place, becausethere is not enough clearance distance between the downstream face ofthe jet plate and the impaction surface. Neither an adhesive nor an oilyor sticky substance may be used to secure the substrate 96 to theupstream face of the collector plate 90, because the substance usedwould interfere with the weighing of the substrate before and after thetest run.

The innovative feature used in this invention to secure the substratefoil 96 onto the collector plate 90 is to ram the substrate foil 96 ontothe collector plate 90 using an interference fit and into the collectorunit 86. In other words, the diameter of the substrate foil, dimension"c" in FIG. 16, is slightly greater than the inside diameter of thecollector plate, dimension "b" in FIG. 13, collector plate 90, and thecollector unit 86. In practice, if dimension "c" is about 0.005 inchesgreater than dimension "b", the interference fit is satisfactory.

To use this principle, a tool, viz, a ram, is made which is merely asolid metal cylinder whose diameter is about 0.005 inches less thandimension "b". After the substrate has been weighed and before the testhas been run, the tool is used to ram the substrate 96 onto thecollector plate 90 such that the substrate is held against the upstreamface of the collector plate 90 and in the collector unit 86. In thismanner the substrate is held firmly in the proper relation to thecollector plate 90 throughout the test, regardless of the position ofthe impactor.

In FIG. 4, it is seen that the length of the passageway 36 and theentrance nozzle 32 is, relatively, long and that the length of the sidewall 52 in the first collector unit 86 is relatively long. Further thelength of the side wall 62 in the second jet stage is not as long as thelength of the passageway 36 in the entrance nozzle 32, but that thelength of the side wall 62 is longer than for succeeding jet stages.Likewise, the length of the side wall 88 in the second collector unit 86is not as long as the length of the wide wall 52 in the first collectorunit 50 but is longer than the side walls in the succeeding collectorunits. One of the reasons for the longer side walls and passageways inthe first few jet stages and collector units is a safety factor to tryto ensure the heavier particles in the aerosol will not be prematurelyimpacted onto a collector plate or onto the walls of the impactor.

After the first few jet stages and collector units, some of thecollector units may have the same wall length. However, in the casewhere some of the jet stages must be made thick, as shown in FIG. 21,the wall lengths may increase as required by the dimensions imposed bythe "step-by-step design process."

The length of the side wall of the collector unit and, the distancebetween the downstream face of the jet plate and the upstream face ofthe next succeeding collector plate cannot be less than the diameter ofthe hole of the jet plate. From experience, I have selected thisdistance to be at least three times the diameter of one hole of the jetplate. It is to be realized that the jet stage, after the first few jetplates, may have the same wall length but have different numbers ofpassageways or jet holes. Further, the diameters of the holes in the jetplates may be different. For example, in the middle portion of theimpactor 30, the diameters of the holes in the jet plates may decreasefor the succeeding jet plates. Then, there is realized a terminaldiameter for the holes in the jet plates as it is not prectical to makea hole any smaller than a certain size in the jet plate. This limitationis dictated by a practical cost limitation for the making of the smallholes in the jet plate. For example, the diameter of the small holes inthe jet plate may be 0.01 inches. For certain jet plates, in the latterstages of the impactor 30, there may be a large number of these smallholes. It may be possible to make holes smaller than 0.01 inches indiameter, but then the number of these holes would be large. The cost ofmaking the holes would be, correspondingly, large. Therefore, from apractical standpoint, the diameter of the holes in the jet plates willbe not less than 0.01 inches. From a theoretical standpoint, the holesin the jet plate can be less than 0.01 inches.

In a typical impactor 30, the number of jet plates will be,approximately, 17 plates. In certain instances, the number of collectorplates may be less than 17 while in other instances, the number ofcollector plates may be much more than 17, such as 30 collector plates.The number of collector plates will be dependent upon the fraction to beseparated and the type of material which is being fractionated. Ingeneral, the design for a soft plastic aerosol would not require as manycollector plates while the design for a hard, dense aerosol wouldrequire, relatively, a large number of collector plates.

In FIG. 4, there is illustrated a jet stage 100. Also, see FIGS. 10, 11,and 12. The jet stage 100 may be considered to be a typical jet stagehaving a circular side wall 110 and a bottom jet plate 112. In thebottom jet plate 112 are a number of holes or passagewyas 114. The sidewall 110 has an inlet face 116 with a circular groove 118 in the inletface 116. Also, the side wall 110 has an outlet face 120 with a circulargroove 122.

The side wall 110 is a tapped passageway 124 for receiving a tap to apressure sensitive device to indicate pressure. In the tapped passageway124, when it is not attached to the pressure sensitive device, is thebolt 70.

Again, it is to be realized that the number of holes or passageways 114and the diameter of these passageways 114 can vary from jet plate to jetplate depending upon the fraction to be separated. A typical collectorunit 86 is illustrated in FIGS. 4, 13, 14 and 15. The collector unit 86comprises a side wall 88 and a bottom collector plate 90 with the holeor passageway 92. There is positioned on the upstream side of thecollector plate 86, the substrate 96 having the hole or passageway 98aligned with the hole or passageway 92.

In FIG. 4, it is seen that in the circular groove 118, there ispositioned an O-ring 134 and in the circular groove 122, there ispositioned an O-ring 136.

In FIGS. 4 and 5, it is seen that the exit fitting 34 has an upstreamface 140 and in the upstream face 140 is a circular groove 142. Also,the central portion of the exit fitting 34 is recessed at 144. In FIG.4, it is seen that in the circular groove 142, there is positioned anO-ring 146.

In the face 44 of the entrance nozzle 32, there are three drilled,tapped passageways 148.

The exit fitting 34 may be considered to have a circular shoulder 150and in the circular shoulder 150 are three passageways 152 which can bealigned with the passageways 148 in the entrance nozzle 32. Three longbolts 154 may pass through the passageways 152 and be screwed into thetapped holes 148 so as to squeeze together the jet stages and thecollectors units to make gastight seals in conjunction with the o-ringsbetween the collector units and the jet stages.

Further, in the exit fitting 34, there is a drilled, tapped passageway156 which is used as a pressure tap. When 156 is not used as a pressuretap, a bolt 70 is screwed into this drilled, tapped passageway 156.

The tapped passageway 40 on the exit fitting 34 is connected to a vacuumpump to cause the aerosol laden gas to flow through the impactor so asto have fractionation of the particulate matter in the aerosol-ladengas. Further, in designing the impactor, it is desirable to know thecapacity of the pump for causing the flow of the gas through theimpactor. It is desirable to know the capacity of the pump as a functionof the downstream pressure of the impactor. By knowing this, it ispossible to know the pump capacity and the minimum size of particulatematter which can be captured by the impactor.

In the entrance nozzle 32, the tapped passageway 36 is designed to beconnected to various inlet tubes so as to achieve isokinetic flow of theaerosol-laden gas at the velocity of the gas stream undergoing tests orbeing sampled.

The impactor 30 may be used in various positions. For example, theimpactor 30 may be used in a horizontal position or it may be used in avertical position with the entrance nozzle 32 pointed into thedownwardly flowing gas stream or the impactor 30 may be used in avertical position with the entrance nozzle 32 pointed in the directionof the upwardly flowing gas stream. Further, in sampling a gas stream,the entrance nozzle 32 is directed into the flowing gas stream so as tosample the gas stream without disturbing the particle size distributionbefore it gets to the impactor.

In FIGS. 18, 19 and 20 there is illustrated a jet stage 170. In FIG. 18,a view looking into the inlet side of the jet stage, it is seen thatthere is a recess 172 for receiving a sealing means such as an O-ring.There is a jet plate 174. In the jet plate 174 are a number of holes orpassageways 176. The holes or passageways 176 are positioned in anequilateral triangle arrangement.

In FIG. 20, a view looking at the outlet side of the jet stage and thedownstream side of the jet plate 174, it is seen that there is a recess178 for receiving a sealing means or an O-ring.

In FIG. 19, a lateral cross-sectional view taken on line 19--19 of FIG.18, it is seen that in the shoulder 179 of the jet stage 170 there is adrilled tapped passageway 180 for receiving a pressure tube.

The jet plate 170 comprises a large number of very small holes such as1/100th of an inch in diameter.

From the foregoing, it is seen that I have provided a cascade impactorwhich makes it possible to sample a gas containing particulates or anaerosol. In sampling the gas or the aerosol, there is used the upstreamstamospheric pressure which is in the range of about one atmosphere ofpressure or about 760 millimeters of mercury at a temperature of 32° F.It is possible to sample a gas or an aerosol at a pressure much lessthan one atmosphere of pressure such as a pressure of 40 Torr or 15Torr. These low pressures of 40 Torr and 15 Torr are realized at highaltitudes and at other places. A Torr is a pressure of one millimeter ofmercury. When the upstream pressure is at atmospheric pressure, thenwith my cascade impactor, it is possible to have a downstream pressureof about 40 Torr.

It is to be emphasized that the pumping system employed with my cascadeimpactor is important. For a portable cascade impactor and for aportable pumping system, it is possible to sample a particle size havinga Stokes (Aerodynamic) diameter of about 40 nanometers, or as small as10 nanometers real diameter for particles of specific gravity equal 3.0.With a more elaborate pumping system, which is not portable, viz.,cannot be carried by one man, it is possible to sample a particle sizehaving a Stokes diameter as small as 10 nanometers. It is to be realizedthat for a dense material or a dense substance a Stokes diameter of 10nanometers may be a real diameter of as small as about 2 nanometers. Forexample, a gold particle having a Stokes diameter of about 10 nanometersmay have an actual diameter of 2 nanometers or less. The upperreasonable limit for particulate matter to be sampled in a cascadeimpactor is at a d₅₀ of about 10,000 nanometers. This upper reasonablelimit is based on the fact that for particle sizes larger than the upperlimit it is extremely difficult to obtain a representative sample of theaerosol.

A novel feature of this invention is that the impactor is designed toexactly match the flow characteristics of a given preselected pumpingsystem. In actual practice, the first thing that is done is to determinethe pressure drop, P, vs. flow, Q, for the vacuum pumping systemselected. See the upper curve in FIG. 28, for a typical such curve.

Then, using the step-by-step design method, the impactor is designed sothat it will operate at design capacity merely by connecting a vacuumhose between the exit of the impactor and the inlet of the vacuumpumping system, and turning on the vacuum pumping system. The impactorinstantly achieves automatic equilibrium of flow at the design flow ratewithout need for manual regulation. The reason for instantself-regulation is seen when one examines the Pressure Drop AcrossImpactor Pump ΔP_(tot) vs Flow Rate Through Impactor Pump, Q.

Referring again to FIG. 28, the total pressure drop across the impactor,P_(tot), increases very rapidly with the increasing flow rate. When thetotal pressure across the impactor, ΔP_(tot), equals the pressure dropgenerated by the vacuum pumping system, ΔP_(pump), the flow rateinstantly stabilizes with the design flow rate and within the impactorachieves design velocities and design particle separation.

There are two important advantages that accure from this self-regulationfeature, namely, (1) operation of the impactor is very simple, requiringonly to turn the switch on and off, and (2) simplicity of design andoperation leads to reliability.

The impactor described in the previous pages is the preferred embodimentof this invention. In fact, a working model of the invention, asdescribed, has been built and tested in actual field conditions.However, other configurations could be designed and constructed whichwould use the teachings of this invention. In particular, let usconsider the passage through which the gas flows from the collectorplate surface of one stage to the upstream face of the jet plate in thenext succeeding stage. This passage must be one having a low, that is,negligible, resistance to the flow of the gas. The preferred embodimentuses a circular hole 92 in the central part of the collector plate 90 toaccomplish this purpose. Some other impactors described in theliterature employ collector plates in which the gas flows from thecollector plate to the next jet plate through passages around theperiphery of the collector plate. FIGS. 7, 9 and illustrate a collectionplate using the latter configuration. The point being made is that theteachings of this invention could be utilized with either center holesor peripheral slots in the collector plates, or indeed still otherconfigurations.

In the terminology there is a reference to d_(m)(j-1). The term "j" isan enumerator which refers to the stage of the impactor underconsideration. The term (j-1)refers to the stage immediately upstreamfrom the stage of the impactor under consideration. The term "d" is thediameter of particle being captured on a given stage. The term "m" isthe percent of particles captured on a given stage. Thus, for instance,d₉₈(j-1), refers to the diameter of particle, 98 percent of which willbe captured on the collector plate of the stage immediately upstreamfrom the stage of the impactor under consideration considered.

THE THEORY AND DESCRIPTION OF DESIGN PROCESS AND IMPACTOR OPERATION

The essence of the design of any round-hole cascade impactor is theselection of the diameter of jet holes and number of jet holes in eachstage of the impactor so as to achieve the particle separation that isdesired, that is, to design the impactor so as to achieve the d₅₀ valuesthat are desired. In the particular case of this invention, the essenceof design is the selection of the diameter of the jet holes, the numberof jet holes, and the thickness of the jet plate, to achieve the desiredparticle separation and to achieve this separation without particlebounce or reentrainment.

THE GENERAL THEORY OF CASCADE IMPACTORS

All modern cascade impactors are based on the Ranz and Wongrelationship: ##EQU1## These investigators found, theoretically andexperimentally, that the probability of a particle of diameter "d",being carried in a jet stream directed toward a flat plate, would beimpacted on said plate, depends upon the value of the impactionparameter, ψ. For example, if a particle has a fifty (50%) percentprobability of being impacted (and captured) on a given collectionplate, said particle diameter would be designated d₅₀, the correspondingvalue of the impaction parameter is called ψ₅₀. Various workers,including the inventor, have found that when a particle has a fifty(50%) percent probability of being captured on a given stage, then thevalue of ψ₅₀ falls between 0.12 and 0.17 (the exact value varies withdifferent experimenters). The inventor has found that a value of ψ₅₀equals 0.145 is appropriate and most nearly fits all of his experimentaldata. The inventor has also experimentally determined various otherspecific values of ψ, such as:

ψ95=0.192 (value of ψ when there is a 95% probability of capture ofparticle of diameter d₉₅)

ψ98=0.209 (value of ψ when there is a 98% probability of capture ofparticle of diameter d₉₈)

ψ100=(approximately) 0.245 (value of ψ when there is a 100% probabilityof capture of particle of diameter d₁₀₀)

In practice, the value ψ₁₀₀ is a limiting value and (1) is verydifficult to determine accurately, and (2) leads to unnecessarilyconservative and cimbersome designs, as will be seen in the followingdiscussion.

The terms of the above equation are found in the nomenclature section.

It is readily seen that if the value of ψ₅₀ is a constant and is known,then value of d₅₀ for the various stages of the impactor can becalculated, and a design predicted. Indeed, this is the method used forprevious cascade impactors.

THE PRESSURE DROP AND TEMPERATURE DROP PREDICTION AS PART OF THE DESIGNMETHOD

In previous impactors, the assumption was made that pressure dropthrough the impactor had a negligible effect upon the impaction process.This assumption is close enough to reality to be able to designimpactors which can size particles as small as 300 nanometers indiameter. However, the Cunningham slip correction factor, C, changesrapidly with decreasing pressure when the d₅₀ values become less than300 nanometers.

An innovative feature of this invention is the precise prediction ofpressure drop and temperature drop across each jet stage. This precisionin prediction of downstream pressures and temperatures make possible thedesign of the impactor that extends the lower limit of particlediameters that may be captured from the present 300 nanometers to about40 nanometers aerodynamic diameter (as small as 10 nanometers realdiameter for dense particles).

Pressure drop across a given stage, P, is calculated by modifying theclassical orifice equation to apply to a plate with more than oneorifice, and solving for pressure drop; thus: ##EQU2##

All of the terms are defined in the nomenclature section of thisspecification. All of the terms in the right side of the above equationmay be readily determined except the orifice coefficient, C_(v). C_(v)is a complex function of jet stream Reynolds number, Rej, and ofthickness-to-diameter ratio of the jet holes, t/D. Theexperimental-mathmatical method of determining C_(v) is explained indetail in the "step-by-step design process".

Once the pressure drop and the downstream pressure for a given stagehave been determined, the temperature drop and the downstreamtemperature are predicted by calculating the adiabatic expansion of agas. The procedure for accomplishing this prediction is described indetail in the "step-by-step design process".

The accurate prediction of pressure drop and temperature drop makespossible the accurate prediction of conditions of impaction of particleswith d₅₀ values as small as desired. There are two practicalconsiderations to be considered in regard to the particle size and theparticle size distribution. First, as the d₅₀ becomes smaller andsmaller, it is necessary for the vacuum pumping system, which causes thegas to flow through the impactor, to pump larger and larger volumes ofgas at lower and lower downstream pressures. Greater and greater pumpingcapacity is associated with greater weight, size and cost of the pumpingmechanism. Usually, it is desired to be able to carry the pumping systemto the testing site. Therefore, a compromise must be made between thesmallest d₅₀ attainable, and size and portability and cost of thepumping system. A large pumping system with a high capacity is desirablebut may not be protable and may not be usable at the site of the test.

The second limitation is that as the d₅₀ values become smaller, the massto be weighed on each substrate becomes less. A factor to consider isthe limit of sensitivity of available balances to weigh the particlesthat are collected. For the purpose of this invention, a Stokes(aerodynamic) diameter of 40 nanometers for the last stage is believedto be the practical limit, that is d₅₀ for the last stage equals 40nanometers Stokes diameter (as small as 10 nanometers real diameter fordense particles).

THE THEORY OF BOUNCE PREVENTION

A particle will not bounce from a surface if the energy that isavailable to hold the particle to the surface is greater than thekenetic energy that is conserved by the particle during collision.

The energy that is available to hold the particle to the substratesurface is the surface energy between the particle and the substrate.This quantitly is very difficult to determine in absolute terms;however, this quantity is proportional to the surface area of theparticle, that is, proportional to πd².

The kinetic energy that is conserved by a particle during collision is,likewise, difficult to determine. However, this quantity is proportionalto the total kinetic energy of the particle in the gas stream, that is,proportional to ##EQU3##

It follows that, if the kinetic energy of the particle is the gasstream, per unit area of the particle, does not exceed some thresholdvalue, the particle will not bounce. This threshold value, is defined asthe Nelson bounce parameter, ##EQU4##

The inventor has found experimentally, that in the case of potassiumsulfate aerosol directed toward an aluminum foil substrate, if the valueof β is less than 350 g/sec², the particles will bounce severly andunpredictably. Therefore, a principal teaching of this invention is todesign an impactor, using Ranz and Wond theory to predict d₅₀ values,and at the same time to keep velocities of the various stages no greaterthan the threshold value of 62 which will cause bounce. It is recognizedthat this threshold value of β will be specific and different for eachcombination of aerosol composition and substrate composition, for tworeasons. Firstly, surface energy per unit area of particle is specificfor each such combination. Secondly, the fraction of kenetic energyconserved is dependent upon the elasticity of particle and elesticity ofthe substrate. It is believed that the combination of potassium sulfateaerosol and aluminum substrate is one of the worst combinations that aninvestigator is likely to encounter as regards tendency to bounce. Itfollow that a value of β=350 g/sec² would lead to a conservative designthat would eliminate bounce for the vast majority of aerosol-substratecombinations.

In using this insight into bounce prevention, one first selects adiameter, d, which will not be presented to the stage in question. Thesmallest diameter that has no possibility of being presented to a stageis the d₁₀₀ diameter for the stage prior to the stage in question.Therefore, in designing the stage, the velocity should be no greaterthan that which will give a value of 350, for example, using the d₁₀₀diameter for the stage prior to the stage in question, that is, β shouldbe no greater than ##EQU5## There is one modification of this procedure;d₉₈ is substituted for d₁₀₀ in the above relationship, that is, thevelocity is limited to that for which β is not greater than ##EQU6##This modification results in a design with fewer stages, at the cost ofhaving a very few particles bounce. At the very worst, no particle thatwould be large enough to bounce would have no more than about a 0.75percent probability of reaching the given plate. It is also probablethat the value d₉₅ could be substituted for d₁₀₀, resulting in stillfewer stages and a more compact design, but also at the additional riskof some bounce for hard particles.

THE ROLE OF VERY THICK JET PLATES IN BOUNCE PREVENTION

The above described theory has been used to design and construct animpactor with values of equal to or less that 350 g/sec². The designprocess dictated that the d₅₀ values, of stages between d₅₀ =400nanometers and d₅₀ =100 nanometers be spaced much closer together thanwould otherwise be desired. This impactor had 15 stages whose d₅₀diameters are as follows:

    ______________________________________                                        Stage             d.sub.50, nanometers                                        ______________________________________                                        1                 16,000                                                      2                 7,400                                                       3                 4,000                                                       4                 1,900                                                       5                 910                                                         6                 420                                                         7                 290                                                         8                 250                                                         9                 230                                                         10                200                                                         11                170                                                         12                130                                                         13                91                                                          14                59                                                          15                45                                                          ______________________________________                                    

Form the point of view of convenience of operation, the d₅₀ of eachstage should be approximately one half of the d₅₀ of the stateimmediately upstream. Such an impactor should need only 9 stages tocover the same range of particle diameters.

The first mentioned impactor had plates with a thickness dimention "t"in FIG. 19, of 0.050 inches for stages with the minimum jet holediameter. If "t" is increased, as shown in FIG. 21, the pressure dropacross the stage is increased without a corresponding increase in thevelocity of the gas on the downstream side of the jet holes. To phraseit another way: (1) the increased length of travel of the gas throughthe jet holes causes the gas and the particles to be slowed down ordecelerated by wall friction losses; (2) thereby achieving the lowpressure and high Cunningham factors needed for impaction; (3) havevelocities low enough to avoid bounce; and, (4) spacing the d₅₀ valuesfar enough apart so that the whole range of particle sizes can be caughtwith about nine stages.

Thus, by using the variable of jet plate thickness, as well as thevariable of number of holes in the jet plate, it becomes possible toobtain simultaneously (1) optimum desired spacing of d₅₀ values and (2)limitation of values of β to avoid bounce. The detailed procedure forusing this concept is incorporated in the "step-by-step design process".

It is of course obvious that it is impossible to protect the first stagefrom bounce by control of the β parameter, because there is no controlover the size of particles that might reach the stage. However, this isnot usually a problem because for the first stage, the velocity neededto cause impaction of the largest particles is so low that the value ofβ is low, that is, the velocity is controlled by considerations otherthan the value of β. Alternatively, the particles larger than about 10microns or 10,000 nanometers can be removed by a precut cyclone justupstream from the first stage of the impactor.

A further consideration is that, since β is proportional to the densityof the particle, ρ_(p), the greater the density of the particle, themore the bounce problem is agravated. Most atmospheric aerosols have adensity of about 1.0 g/cm³ and even higher.

To design an impactor which will have the widest utility and freedomfrom bounce problems, one should select a value of ρ_(p) whichrepresents a worst case condition. The combination of potassium sulfateaerosol (sp.gr=2.66) and an aluminum foil substrate is believed to beabout the worst case from a bounce point of view, that one wouldencounter in practice, because the aerosol is a hard particle contactingan eleastic metal substrate and because the particle has a relativelyhigh density.

OTHER IMPORTANT DESIGN CONSTRAINTS

A principal teaching of this invention is to limit the value of thebounce parameter so as to eliminate bounce. However, if this were theonly constraint, one might have an impactor having turbulent flow aroundthe particles, and/or supersonic flow from the jets would result.Therefore, the design process incorporates two additional limitations.First, the jet stream Reynolds number must be limited to 3200, i.e., tokeep gas flow around the particles in the viscous region. This limit onthe Reynolds number is a result of the Ranz and wong theory, upon whichthis work depends, is predicted upon viscous flow around the particles.Cohen and Montan (1967) developed the theory that states that gas flowaround the particles will be in the viscous region if the gas flowthrough the jets has a Reynolds number not greater than 3200. Fromexperience, I consider that it is desirable to have a jet Reynold numberno greater than about 1200. Therefore, I have limited the jet Reynoldsnumber to not greater than about 1200.

A second limitation is the Mach number limited to 1.0 A Mach numbergreater than 1.0 is not possible unless an expanding nozzle is used inthe jets. Such a nozzle is impractical to make, and, in addition, thereis no available theory with which to calculate the impaction ofparticles from such a nozzle. In practice, very little is gained byhaving velocities approaching 1.0. The inventor has found it possible tomake a very good design by limiting the Mach number to 0.8.

ELECTROPOLISHING TO ROUND THE UPSTREAM EDGES OF JET HOLES

The entrances to the jet holes 176 may be sharp square edges 177, asshown in FIG. 23, or they may have rounded edges 184 as shown in FIG.24. As far as is known, all previous impactors have used sharp squareedges 177 on jet holes having a diameter smaller than 0.050 inches. Ifthe edges are sharp and square, the abrupt change in velocity of the gasas it enters the jet hole 176 creates some turbulence at the entrance,which in turn cause some of the particulate matter 182 to be prematurelydeposited on the upstream face of the jet plate 170. FIG. 22 is acutaway fragmentary section of the face of a jet plate 170, showingunwanted deposits of particulate matter 182 around the entrances to theholes 176. FIG. 23 is a vertical section across FIG. 22 along line23--23 and shows the unwanted accumulation of particulate matter 182 andalso shows the patterns of tubulent gas flow near the entrance to thehole. By contrast, FIG. 24 shows the smooth air pattern around theentrance to the hole when the entrance is rounded.

For the larger jet hole sizes, countersinking of entrances of jet holeshas been described for previous impactors, to avoid this problem ofturbulence at the jet hole entrances. However, for hole sizes smallerthan about 0.050 inches in diameter, mechanical countersinking isdifficult. And, yet the hole sizes on more than half of the jet stagesof the present impactor must be smaller than 0.050 inches in diameter inorder to make this impactor functional.

An innovative feature of this invention is that the upstream edges ofjet holes smaller than 0.050 inches diameter are rounded byelectropolishing. Electropolishing is a term used by industry todescribe the process by which a workpiece is polished by making it theanode in a suitable electrolyte and passing through the part, directelectricla current at a high current density. This process removes metalfrom the workpiece. The nature of the process is such that edges andprotuberances are removed at a faster rate than the metal from the flatpart of the workpiece. The relative positions of the workpiece and theother electrical parts during electropolishing, are shown in FIG. 25.The workpiece, a jet stage, is connected to the anode of positive orplus (+) side of the current source. The circuit is completed by placingthe cathode very close to but not touching the workpiece and connectingit with the negative or minus (-) side of the current source. Theworkpiece and the cathode are then immersed in a suitable electrolyte ata suitable temperature.

The choice of electrylyte depends upon the metal that constitutes theworkpiece. For aluminum alloys, a solution of 25 grams/liter offluoboric acid at 86° F. works well. For stainless steel, a solution ofsulfuric acid, citric acid and methyl alcohol at 130° F. is verysatisfactory. The reference Tegart (1956) states the necessary detailsconcerning electropolishing.

Chemical polishing, that, the polishing by immersion in a chemicalsolution without the use of an externally applied electromative force orelectrical current, is a possible but less satisfactory way to round theentrances to the jet holes. Techniques for chemical polishing are alsodescribed by Tegart (1956).

By way of summation, electropolishing or chemical polishing affords away to round the entrances to even the smallest jet holes and to preventunwanted deposition of particulate on the upstream faces of the jetplates.

OVERVIEW OF THE DESIGN PROCESS

The teachings of this invention employ several principles which areinterdependent. The design process is a systematic and logicalorganization of these principles and incorporates both experimental andtheoretical steps to arrive at a design which will accomplish theobjectives of this invention.

The simplest statement of the design process is as follows: The designerchooses the initial design conditions, that is, the number of stages thed₅₀ diameter for each stage, initial gas temperature, initial gaspressure, gas flow rate, particle density, maximum jet Reynolds number,and minimum drill size for the jet holes. The design process thenselects (1) the number of jet holes in each stage, (2) the diameter ofthe jet holes in each stage, and (3) the length of the jet holes in eachstage, i.e., the thickness of the jet plates and the dimension "t" asillustrated in FIGS. 19 and 21. A detailed description of the designprocess follows.

THE STEP-BY-STEP DESIGN PROCESS

A step-by-step design process is presented in the following outline.

1. Select the initial design conditions.

a. The designer determines the number of stages and d₅₀ value for eachstage. A practical general purpose design has a d₅₀ diameter of 10,000nanometers for the first stage. then each succeeding state has a d₅₀diameter one-half the diameter of the preceding stage. This criteriawill yield an impactor which will cover the range from 10,000 nanometersto 40 nanometers in nine stages. There is an infinite number ofcombinations of stages and d₅₀ values that may be selected forparticular purposes.

b. The designer selects the initial temperature for the inlet. Thistemperature is usually 70° F., but any reasonable temperature may beused.

c. The designer selects the inlet pressure. A satisfactory inletpressure value may be less if one were designing the impactor for use ata high altitude.

d. Flow rate of aerosol through the impactor. This quantity is ofnecessity a comprimise between a high flow rate wanted for securing arepresentative sample, vacuum pumping system capacity when at very lowpressures downstream, the need for low downstream pressure to getimpaction of very small particles, and a vacuum pumping system that isportable enough to be carried to a test side. A reasonable comprimisehas been a flow rate of 0.35 cfm (measured at inlet conditions) withwhich can be attained a downstream pressure of 40 Torr, and with whichcan be impacted a particle of 40 nanometers Stokes diameter.

e. Density of particle to be impacted. Select a value of 2.5 g/cm³ tocover the probable worst case condition, unless the impactor is of aspecial purpose design for which no particles of greater than 1.0 g/cm³will be impacted.

f. Maximum jet stream Reynolds number. A value of 1200 recommended.

g. Minimum drill size. A value of 0.-10 inches is recommended. If therewill be several stages with very thick jet plates, a larger value ofD_(min) may be advisable.

2. Empirically derive the constants required for determination of C_(v)in the pressure drop equation.

a. Prepare several test jet plate stages. Each stage should have about70 holes of equal diameter. At least five test stages are required,differing from one another only in the length of the holes (that is, jetplate thickness) and/or in the diameter of the holes. The jet holediameters should be in the range of 0.010 inch to 0.0135 inch, the platethicknesses should be in the range of 0.050 inch to 0.50 inch, and thet/D ratio should cover the range between 3 to 35.

b. Electropolish the upstream edges of the test stages if the designwill use the electropolished jet holes. In any event, all the teststages should be treated alike, viz., all electropolished or noneelectropolished.

c. Connect test stages for measurement of pressure drop across the stageas a function of flow rate. Connect equipment with flexible hoses in thefollowing order, from upstream to downstream. (1) dry gas meter, (2)test stage, (3) throttling clamp or valve and (4) vacuum pump. Connect amonometer with one tap between the gas meter and the test stage and theother tap between the test stage and the throttling clamp.

d. Make pressure drop measurements. Turn on vacuum pump and regulateflow with the throttling clamp. At a number of throttling clampsettings, record the flow rate and the pressure drop across the jetstage. For each test stage, about 20 pairs of observations aresuggested, covering Reynolds numbers between 100 and 1500. Repeat stepsc. and d. for each of the several test stages.

e. For each observation, calculate Reynolds number: ##EQU7##

f. For each observation calculate the orifice coefficient: ##EQU8##

g. Using the data developed in steps a. through f., evaluate theempirical constants K₁, K₂, K₃, K₄, and K₅, in the equation:

    C.sub.v =K.sub.1 +K.sub.2 Rej.sup.2 +K.sub.4 (t/D)+K.sub.5 (t/D).sup.2

The evaluation may be conveniently done with a stepwise multipleregression computer program, such as the BMDO2R, described by Dixon(1968).

3. For the first stage set N=1.

4. Note the value of d₅₀ which has been preselected for the first stage.5. Calculate D for the first stage: ##EQU9## Readjust D to equal thediameter of the nearest commercially available drill size.

6. Set t=D.

7. Calculate d₉₈ for the first stage:

    d.sub.98 =d.sub.50 √ψ.sub.98 /ψ.sub.50

8. Calculate jet stream Reynolds number: ##EQU10##

9. Calculate orifice coefficient. If D is greater than 0.016 inch,

    C.sub.v =(0.8657+(0.00002232)(Rej)-(2.752×10.sup.-8)(Rej).sup.2 +(5.633×10.sup.-12)(Rej).sup.3 -(0.01731)(t/D).sup.2 +(0.0007474)(t/D).sup.3 +(0.00007563)(Rej)(t/D)-(3.005×10.sup.-6)(Rej)(t/d).sup.2 -(1.197×10.sup.-8)(Rej).sup.2 (t/D))/(p/D).sup.0.1

If D is equal to or less than 0.016 inch,

    C.sub.v =K.sub.1 +K.sub.2 (Rej)+K.sub.3 (Rej).sup.2 +K.sub.4 (t/D)+K.sub.5 (t/D).sup.2

10. Calculate the pressure drop across the stage. ##EQU11##

11. Calculate downstream pressure.

    P.sub.d =P.sub.u -ΔP

12. Calculate downstream temperature. The following calculation assumesthe adiabatic expansion of a perfect gas; the calculation was developedfrom the exposition of Shapiro (1954):

a. Calculate Mach number of gas stream at entrance to jet hole(s):##EQU12##

b. Calculate first intermediate variable:

    A=(S.sub.4 -1)/2

c. Calculate second intermediate variable:

    B=(P.sub.u Ma.sub.u /P.sub.d).sup.2 (1+A Ma .sub.u.sup.2)

d. Calculate the Mach number of gas stream at exit to jet holes(s):

    Ma.sub.d =((-1+(1+4A B).sup.0.5))/2A).sup.0.5

e. Caclulate downstream temperature:

    T.sub.d =T.sub.u (1+A Ma.sub.u.sup.2)/(1+AMa.sub.d.sup.2)

13. Note the value of d₅₀ that has been preselected for the next stage.Proceed to calculate P, N, and t for the next stage.

14. Calculate D.

    D=12.74d.sub.50 ((C.sub.50 ρ.sub.p Rejt T.sub.u)/(ψ.sub.50 P)).sup.0.5

Readjust D to equal the diameter of the nearest commercially availabledrill size.

15. Set t=D, or t=0.05 inch, whichever is the greater.

16. Calculate trial number of holes for the stage in question:

    N=(0.899×10.sup.8 d.sub.50.sup.2 ρ.sub.p WT.sub.u.sup.0.232)/(D.sup.3 ψ.sub.50 P.sub.u)

Round off N to the nearest integer.

17. Calculate Reynolds number as in step 8.

18. Calculate orifice coefficient, pressure drop, downstream pressure,and downstream temperature as in steps 9, 10, 11, and 12.

19. Recalculate N on the basis of downstream conditions:

    N=(0.899×10.sup.8 d.sub.50.sup.2 ρ.sub.p WT.sub.d.sup..232)/(D.sup.3 ψ.sub.50 P.sub.d)

Round off N to the nearest integer.

20. Recalculate Rej on the basis of downstream conditions:

    Rej=(0.5535×10.sup.-6 W)/(NDT.sub.d.sup.0.768)

21. Repeat steps 18, 19, and 20, until two successive values of N areequal.

22. Calculate the velocity of gas at exit to jet holes:

    V=(11692WT.sub.d)/(πD.sup.2 P.sub.d N)

23. Calculate maximum velocity of gas through jet holes so thatparticles will not bounce, that is, so that the bounce parameter, β,will not be exceeded:

    V.sub.max =((12β)/(ρ.sub.p d.sub.98(j-1))).sup.0.5

24. Calculate d₉₈.

    d.sub.98 =d.sub.50 ((C.sub.50 ψ.sub.98)/(C.sub.98 ψ.sub.50)).sup.0.5

25. If V is equal to or less than V_(max), go to step 13. If V isgreater than V_(max) and D is greater than D_(min), set D equal to thenext smaller commercially available drill size and go to step 15. If Vis greater than V_(max) and D equals D_(min), go to step 27.

26. Note the preselected value of d₅₀ for the next stage. Set t at aninitial value of 0.04 inch. Proceed to calculate N and t for the nextstage.

27. Add .01 inch to the current value of t.

28. Calculate trial number of holes for the stage in question, as instep 16.

29. Calculate Reynolds number as in step 8.

30. Calculate orifice coefficient, pressure drop, downstream pressure,and downstream temperature as in steps 9, 10, 11, and 12.

31. Recalculate N on the basis of downstream conditions as in step 19.

32. Recalculate Reynolds number on the basis of downstream conditions asin step 20.

33. Repeat steps 30, 31, and 32, until two successive values of N areequal.

34. Calculate velocity of gas at exit to jet holes as in step 22.

35. Calculate maximum velocity of gas from jet holes so as not exceedbounce parameter, as in step 23.

36. Calculate d₉₈ as in step 24.

37. If V is greater than V_(max) go to step 27. If V is equal to or lessthan V_(max) and the stage in question is not the last stage, go to step26. If V is equal to or less than V_(max) and the stage in question isthe last stage, go to step 38.

38. Measure the total pressure drop as a function of flow rate for thevacuum pumping system that will be used with the impactor:

a. Connect equipment for test with flexible hoses in the following orderfrom upstream to downstream. (1) dry gas meter, (2) throttling valve (3)vacuum pumping system. Connect a manometer with one tap between thethrottling valve and the vacuum pumping system and the other tap exposedto the ambient air.

b. Close the throttling valve, turn on the vacuum pumping system, andrecord the pressure drop at zero flow rate. Open throttline valveslightly and record pressure drop and flow rate. Repeat with about adozen valve settings from maximum pressure drop to about 600 Torr.

c. Using the above data, evaluate the constants K₆ and K₇ in theequation:

    ΔP.sub.pump =K.sub.6 +K.sub.7 Q

See FIG. 28 for illustration of the curve to be expected.

39. Using the relationship developed in step 38, calculate ΔP_(pump) forthe flow rate that was chosen for the design in question.

40. If the total pressure crop across the impactor, ΔP_(tot), is greaterthan ΔP_(pump), then (1) decrease assumed flow rate through the impactoror (2) increase the d₅₀ of the last stage, and go to step 3. If ΔP_(tot)is less than ΔP_(pump), then (1) increase assumed flow rate through theimpactor or (2) decrease the d₅₀ of the last stage and go to step 3.

41. Repeat steps 3 through 40 until ΔP_(tot) and ΔP_(pump) agree within±2 Torr.

USE OF JET SLITS INSTEAD OF ROUND JET HOLES

Cascade impactors are sometimes made using jet slits instead of roundjet holes. FIG. 29 shows a typical jet stage wherein the jet passagewaysare rectangular slits instead of round holes. FIG. 30 is across-sectional view of FIG. 29. Note the jet slit(s) 200. The slits donot need to be rectangular; but may be annular or otherwise curvilinear.The only requirement is that all the slits in a given stage must havethe same uniform width.

    __________________________________________________________________________    NOMENCLATURE                                                                  __________________________________________________________________________    A.sub.f   Total cross sectional area of hole(s) or slit(s) in a given                   stage (cm.sup.2).                                                   A.sub.t   Total cross sectional area of the inside of the impactor                      (cm.sup.2).                                                         A,B       Intermediate variables in temperature drop calculation; see                   step 12.                                                            C         Cunningham slip correction factor (dimensionless).                   C =                                                                                     ##STR1##                                                           C.sub.50  Cunningham factor corresponding to d.sub.50                         C.sub.98  Cunningham factor corresponding to d.sub.98                         d         Aerodynamic diameter of particle being impacted (cm, except                   nanometers                                                                    when so noted).                                                     d.sub.50  Particle diameter, 50% of which will be impacted on a given                   stage.                                                              d.sub.98  Particle diameter, 98% of which will be impacted on a given                   stage.                                                              D         Diameter of jet hole (cm, except inches where so noted).            D.sub.min Minimum diameter of jet hole assumed for a given stage.             j         Stage enumerator, that is, j is the stage being considered; j -               1 is the                                                                      stage immediately upstream from the stage being considered.         K.sub.1,K.sub.2, K.sub.3, K.sub.4, K.sub.5                                              Empirically determined constants in the equation for orifice                  coefficient for round jet holes.                                    K.sub.6,K.sub.7                                                                         Empirically determined constants in the equation for pump                     performance.                                                        K.sub.8,K.sub.9,K.sub.10,K.sub.11,K.sub.12                                              Empirically determined constants in the equation for orifice                  coefficient for slit jets.                                          L         Total length of slit(s) in a given stage (cm).                      L.sub.1   Length of a single slit in a given jet stage.                       Ma        Mach number, that is, the ratio of the velocity of the gas to                 the vel-                                                                      ocity of sound at the same temperature and pressure                           (dimensionless).                                                    Ma.sub.u  Mach number at the entrance to the jet hole(s) or slits of a                  given stage.                                                        Ma.sub.d  Mach number at the exit to the jet hole(s) or slits of a given                stage.                                                              Mw        Molecular weight of the gas (grams per mole).                       N         Number of round holes in a given jet plate.                         p         Pitch, that is, center-to-center distance between adjacent                    holes on a                                                                    jet plate (cm).                                                     P         Absolute static gas pressure (g/cm.sup.2, except Torr where so                noted).                                                             P.sub.u   Pressure upstream from a given stage.                               P.sub.d   Pressure downstream from a given stage.                             ΔP  Pressure drop across a stage.                                       ΔP.sub.tot                                                                        Total pressure drop developed across the entire impactor.           ΔP.sub.pump                                                                       Total pressure drop developed by the vacuum pumping system.         Q         Volumetric flow rate through the impactor and/or pump (cm.sup.3               /sec,                                                                         expressed at inlet conditions to impactor or outlet conditions                to                                                                            pump).                                                              Rej       Reynolds number for the gas stream in a given jet plate with                  round                                                                         jet hole(s). See step 8. (dimensionless)                            Rejt      Target maximum Reynolds number for a given stage.                   Res       Pseudo-Reynolds number for the gas stream in a given jet stage                with                                                                          slit jets. See substitute step 8 in slit jet design                           (dimensionless).                                                    S.sub.r   Specific heat ratio for the gas (dimensionless). S.sub.r =                    1.403 for air.                                                      t         Length of the jet hole(s) or depth of the jet slit (cm). For                  jet holes                                                                     or slits without countersinking or chamfering, t equals the                   thickness                                                                     of the jet plate. For jet holes with countersinking, t equals                 the                                                                           length of the cylinderical portion of the hole. For jet slits                 with                                                                          chamfering, t equals the depth of the section of the slit which               has                                                                           parallel sides.                                                     T         Temperature of the gas stream (° K., except ° F.                where so notes).                                                    T.sub.u   Gas temperature upstream from a given stage.                        T.sub.d   Gas temperature downstream from a given stage.                      V         Velocity of gas (cm/sec).                                           V.sub.max Maximum velocity of gas so as not to exceed bounce parameter.       W         Mass rate of flow of gas (grams/sec).                               Wi        Width of slit for a given slit jet stage (cm).                      Wi.sub.min                                                                              Minimum width of slit assumed for a given design.                   Y         Gas expansion factor (dimensionless).                                Y =                                                                                     ##STR2##                                                            β                                                                                  ##STR3##                                                           ρ.sub.g                                                                             Density of gas (g/cm.sup.3).                                        ρ.sub.p                                                                             Density of particle being impacted (g/cm.sup.3).                    μ      Viscosity of gas at given temperature (g/sec-cm).                    ψ                                                                                   ##STR4##                                                            ψ.sub.s                                                                             ##STR5##                                                           ψ.sub.50                                                                            Impaction parameter corresponding to d.sub.50 for round jets.       ψ.sub.98                                                                            Impaction parameter corresponding to d.sub.98 for round jets.       ψ.sub.50s                                                                           Impaction parameter corresponding to d.sub.50 for slit jets.        ψ.sub.98s                                                                           Impaction parameter corresponding to d.sub.98 for slit              __________________________________________________________________________              jets.                                                           

REFERENCES

J. J. Cohen and D. M. Montan (1967) "Theoretical considerations, designand evaluation of a cascade impactor," Am. Ind. Hyg. Ass. J., 28,95.

W. J. Sixon, (1968)"Biomedical computer programs", University ofCalifornia Press.

P. A. Nelson, (1973) "A high pressure drop cascade impactor for sizingparticles between 0.03 microns and 10 microns in diameter," M.S.E.Thesis, University of Washington, Seattle.

W. J. McG. Tegart (1956) "The Electrolytic and Chemical Polishing ofMetals", Pergamon Press, London.

W. J. Ranz and J. B. Wong, (1952) "Impaction of dust and smoke particleson surface and body collectors", Inc. Eng. Chem 44 1371

O. H. Shapiro, (1954) "Dynamics and thermodynamics of compressible fluidflow,"Ronald Press, New York.

    ______________________________________                                                    Date        Patent Number                                         ______________________________________                                        Andersen, Ariel A.                                                                          Septemper, 1961                                                                             3,001,914                                         Lasseur, Claude                                                                             September, 1970                                                                             3,528,279                                         Pilat, Michael J.                                                                           September, 1972                                                                             3,693,457                                         Klingler, George A.                                                                         November, 1973                                                                              3,771,291                                         Andersen, Ariel A.                                                                          March, 1974   3,795,135                                         ______________________________________                                    

W. P. Holland, R. E. Conway; Three Multi-Stage Stack Samplers, ChemicalEngineering Progress, Volume 69, No. 6, Pages 93-95

I claim:
 1. A cascade impactor for sampling a particle laden gas forparticle size distribution, said impactor comprising:a. a gas inlet anda gas exit; b. a plurality of stages, each stage comprising a jet plateand a collector plate except that the first jet plate may be a part ofor may be replaced by the inlet nozzle; c. a collector plate downstreamfrom each jet plate, and a jet plate upstream from each collector plate;d. each jet plate having one or more hole(s) through which the particleladen gas passes wherein said hole(s) are positioned so that the gasfrom the jet plate is directed to the collector surface on therespective collector plate downstream from said jet plate; e. a fluidflow path between each collector plate and the jet plate immediatelydownstream and which path allows relatively unrestrained flow of theparticle laden gas from each collector plate to the next succeeding jetplate; f. appropriate sealing means between a jet plate and a collectorplate to cause the gas to flow entirely through the jet plate holes andthrough the designated passageways between collector plate andsucceeding jet plate; and, one or more said jet plates vary in thicknessand in which jet plates the jet hole is less than 0.10 inch in diameterand in which the ratio of the length of a jet hole to the diameter ofsaid jet hole to the diameter of said jet hole is greater than 8, andexpressed as t/D is greater than 8, and where t is defined as the lengthof the cylindrical section of the jet hole and for square edged holes, tequals the thickness of the jet plate and with the edges of the jet holecountersunk t is less than the thickness of the jet plate and D is equalto the diameter of the cylindrical section of the jet hole.
 2. A cascadeimpactor according to claim 1 wherein the ratio of the length of any jethole to the diameter of said jet hole is greater than 10, that is, t/Dis greater than
 10. 3. A cascade impactor according to claim 1 whereinthe ratio of the length of any jet hole to the diameter of said jet holeis greater than 13, that is, t/D is greater than
 13. 4. A cascadeimpactor according to claim 1 wherein the ratio of the length of any jethole to the diameter of said jet hole is greater than 16, that is, t/Dis greater than
 16. 5. A cascade impactor according to claim 1 whereinthe ratio of the length of any jet hole to the diameter of said jet holeis greater than 20, that is, t/D is greater than
 20. 6. A cascadeimpactor accoring to claim 1 in which the passageway for affordingunrestrained flow of gas from a collector plate to the next succeedingjet plate, is a central hole in the collector plate.
 7. A cascadeimpactor according to claim 1 and comprising:a. each collector surfacebeing covered with a metallic foil; and, b. said foil being held inplace within the collector plate by interference fit wherein the outerdiameter of the foil is slightly larger than the inner diameter of thecollector plate, and the foil is held firmly in place during thesampling period, by having been forced into the collector plate.
 8. Acascade impactor according to claim 1 wherein the numbers and sizes ofholes in the various jet states are such that with the impactorconnected with a predetermined vacuum pumping system and free of athrottling device and appreciable pressure drop between the exit of theimpactor and the vacuum pumping system, the impactor will automaticallyoperate at its design flow rate to produce the desired d₅₀ separationdiameters.
 9. A cascade impactor for sampling a particle laden gas forparticle size distribution, said impactor comprising:a. a gas inlet anda gas exit; b. a plurality of stages, each stage comprising a jet plateand a collector plate except that the first jet plate may be a part ormay be replaced by the inlet nozzle; c. a collector plate downstreamfrom each jet plate, and a jet plate upstream from each collector plate;d. each jet plate having one or more hole(s) through which the particleladen gas passes wherein said hole(s) are positioned so that the gasfrom the jet plate is directed to the collector surface on therespective collector plate downstream from said jet plate; e. a fluidflow path between each collector plate and the jet plate immediatelydownstream and which path allows relatively unrestrained flow of theparticle laden gas from each collector plate to the next succeeding jetplate; f. appropriate sealing means between a jet plate to cause the gasto flow entirely through the jet plate holes and through the designatedsucceeding jet plate; and, g. said jet plates vary in thickness and inwhich jet plates the jet hole is less than 0.050 inch in diameter andthe upstream edges of the hole(s) are rounded and flared out to effect agradual acceleration of the gas into the jet hole, such rounding beingaccomplished by electropolishing by placing the jet plate in a suitablesolution and making the jet plate the anode in a direct current circuit,at a high current density.
 10. A cascade impactor according to claim 9wherein the rounding of the edges of the jet holes is accomplished byplacing the jet plate in a suitable chemical solution without theimposition of an electric current.
 11. A cascade impactor comprising:a.a plurality of stages of jet plates and collector plates; b. said jetplates and said collector plates alternating with each other; c. a stagecomprising a jet plate and a collector plate wherein said jet plate ispositioned in front of said collector plate; d. a jet plate having afirst surface and a second surface; e. said first surface and saidsecond surface being on opposite faces of the jet plate; f. a peripheralledge on said first surface; g. a first groove in said peripheral ledgefor receiving a first sealing means; h. a first sealing means in saidfirst groove; i. a second groove in said second surface for receiving asecond sealing means; j. a second sealing means in said second groove;k. said jet plate having a passageway; l. a collector plate having acentral passageway; m. said collector plate having a first surface forcontacting said second sealing means of the immediately preceeding jetplate in said stage and having a second surface for contacting the otherfirst sealing means of the immediately succeeding jet plate in thefollowing stage; n. said collector plate having a circumscribingshoulder for contacting said second sealing means and a second surfacefor contacting said first sealing means; o. said circumscribing shoulderbeing of a dimension to space the exit surface of said jet plate and theinlet surface of the collector plate a distance at least the diameter ofthe passageway in the jet plate; p. said jet plate having a shoulderoutside of said ledge and said first groove and said second groove inaligning said jet plate and said collector plate; q. said circumscribingshoulder being of less diameter than said shoulder of said jet platethereby making it possible to nest said collector plate with said jetplate for aligning said jet plate and said collector plate; and, r.means to hold said collector plates and said jet plates in an assembledstate to form said cascade impactor.
 12. A cascade impactor for samplinga particle laden gas for particle size distribution, said impactorcomprising:a. a gas inlet and a gas outlet; b. a plurality of stages,each stage comprising a jet plate and a collector plate except that thefirst jet plate may be a part of or may be replaced by the inlet nozzleand the jet plates in those stages past the first stage vary inthickness with respect to each other; c. a collector plate downstreamfrom each jet plate, and a jet plate upstream from each collector plate;d. each jet plate having one or more hole(s) through which the particleladen gas passes wherein said hole(s) are positioned so that the gasfrom the jet plate is directed to the collector surface on therespective collector plate downstream from said jet plate; e. a fluidflow path between each collector plate and the jet plate immediatelydownstream and which path allows relatively unrestrained flow of theparticle laden gas from each collector plate to the next succeeding jetplate; f. appropriate sealing means between a jet plate and a collectorplate to cause the gas to flow entirely through the jet plate holes andthrough the designated passageways between collector plate andsucceeding jet plage; and, g. said cascade impactor having thecharacteristics that the value of the bounce parameter, β, never exceeds300 g/sec², where: ##EQU13## ρp=specific gravity of particles beingimpacted, g/cm³ d_(m)(j-1) =diameter of particle which has an m percentprobability of being captured on the stage immediately upstream from thestage being considered, cm V=average velocity of the jet stream at thepoint of exit of the stream from the jet plate of the stage beingconsidered, cm/sec.
 13. A cascade impactor according to claim 12 whereinm is equal to 98% probability of being captured.
 14. A cascade impactoraccording to claim 13 wherein β never exceeds 400 g/sec².
 15. A cascadeimpactor according to claim 13 wherein β never exceeds 500 g/sec².
 16. Acascade impactor according to claim 13 wherein β never exceeds 600g/sec².
 17. A cascade impactor according to claim 13 wherein β neverexceeds 800 g/sec².
 18. A cascade impactor according to claim 13 whereinβ never exceeds 1000 g/sec².
 19. A cascade impactor for sampling aparticle laden gas for particle size distribution, said impactorcomprising:a. a gas inlet and a gas exit; b. a plurality of stages, eachstage comprising a jet plate and a collector plate except that the firstjet plate may be a part of or may be replaced by the inlet nozzle; c. acollector plate downstream from each jet plate, and a jet plate upstreamfrom each collector plate; d. one or more of said jet plates having oneor more square or rectangular slits through which the particle laden gaspasses, such slit being positioned so that the gas from the jet plate isdirected to the collector surface on the respective collector platedownstream from said jet plate; e. a fluid flow path between eachcollector plate and the jet plate immediately downstream and which pathallows relatively unrestrained flow of the particle laden gas from eachcollector plate to the next succeeding jet plate; f. appropriate sealingmeans between a jet plate and a collector plate to cause the gas to flowentirely through the jet plate holes and through the designatedpassageways between collector plate and succeeding jet plate; and, g.one or more said jet plates vary in thickness in which jet plates thejet slit is less than 0.10 inch in width and in which the ratio of thedepth of a jet slit to the width of said jet slit is greater than 8, andexpressed at t/Wi is greater than 8 and where t is defined as the depthof the section of the jet slit that has parallel sides and for squareedged slits, t equals the thickness of the jet plate and with the edgesof the slits chamfered, t is then the thickness of the jet plate and Wiis equal to the width of the section of the jet slit that has parallelsides.
 20. A cascade impactor according to claim 19 wherein the ratio oft/Wi is greater than
 10. 21. A cascade impactor according to claim 19wherein the ratio t/Wi is greater than
 13. 22. A cascade impactoraccording to claim 19 wherein the slits have a constant width, Wi, andhave a curvilinear configuration.
 23. A method for determining particlesizes of particular matter in gas in the atmosphere, said methodcomprising:a. varying the direction of the velocity of the gas and theparticulate matter in the jet stream to separate the particulate matterfrom gas to form separated particulate matter; b. collecting theseparated particulate matter and weighing the separated particulatematter; c. repeating the varying of the direction and the velocity ofthe gas and the particulate matter to separate different sizes ofparticulate matter; d. controlling the velocity of the gas and theparticulate matter to have a value for the bounce parameter, β, lessthan 300 g/sec², wherein: ##EQU14## ρp=specific gravity of particlesbeing impacted, g/cm³ d_(m)(j-1) =diameter of a particle which has an mpercent probability of being captured on the stage immediately upstreamfrom the being considered, cm V=average initial velocity of the jetstream at the stage being considered, cm/sec; and, e. causing the gas toflow in jet passageways with the length to diameter ratios greater thaneight, thereby causing the static pressure of the gas downstream fromthe passageways to be reduced thereby causing the cunningham slipcorrection factor to be increased, thereby causing the velocity requiredfor impaction of a given diameter particle to be reduced, therebycausing the 62 (beta) bounce parameter to be reduced to a level suchthat bounce of the particles will not occur.